The subpoena compels him to testify before a grand jury in Mueller's ongoing investigation into possible collusion between the Trump campaign and the Russian government to sway the 2016 presidential election.

Is he legally allowed to refuse to testify?

The short answer is no. The longer answer, according to FindLaw, is: not without a legitimate legal reason.

Refusal to comply with a subpoena can land you in jail with a contempt of court charge. After all, Subpoena literally translates to “under penalty” in Latin.

It sounds like Nunberg is aware of the penalty. He told The Washington Post, "Let him arrest me."

But what are the legal reasons to not testify?

Privilege

Spousal privilege – Spouses cannot be compelled to testify to communications with the other that occurred during the marriage.

Attorney-client privilege – Attorneys cannot be forced to testify about communication with a client.

Doctor-patient confidentiality – A doctor cannot reveal private medical information without the consent of the patient.

Fifth Amendment

The Fifth Amendment protects citizens from testifying if it will reveal self-incriminating information. If you’re not hiding criminal activity, the Fifth Amendment’s protections do not apply.

U.S. President Donald Trump's nominee to head the U.N. migration agency acknowledges a controversy over his social media posts about Muslims and other topics, but insists "retweets are not endorsements" and says he...

U.S. President Donald Trump's nominee to head the U.N. migration agency acknowledges a controversy over his social media posts about Muslims and other topics, but insists "retweets are not endorsements" and says he wants to be judged on his actions.

(AP Photo/Evan Vucci, File). FILE - In this March 15, 2018, file photo, President Donald Trump talks with reporters during a meeting in the Oval Office of the White House in Washington. Trump is questioning the impartiality of Robert Mueller's investig...

The president has long been frustrated by the lengthy and intensifying probe and insists his 2016 presidential campaign did not collude with Russia to influence the election in his favor.

The president has long been frustrated by the lengthy and intensifying probe and insists his 2016 presidential campaign did not collude with Russia to influence the election in his favor.